<!DOCTYPE html>
<html>
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
  <style>
    body {margin: 0;overflow: hidden;}
    #canvas {background-color: antiquewhite;}
  </style>
</head>
<body>
  <canvas id="canvas"></canvas>
  <script>
    /** @type {HTMLCanvasElement} */
    const canvas = document.querySelector('#canvas');
    canvas.width = window.innerWidth;
    canvas.height = window.innerHeight;
    const ctx = canvas.getContext('2d');

    /*
      投影
        位置:   shadowOffsetX, shadowOffsetY
        模糊度: shadowBlur
        颜色:   shadowColor
    */
   ctx.beginPath();
   ctx.arc(300,200,100,0,Math.PI*2);
   ctx.fillStyle = '#93abff';
   ctx.shadowColor = '#000';
   ctx.shadowOffsetX = 10;
   ctx.shadowOffsetY = 10;
   ctx.shadowBlur = 30;
   ctx.fill();
  </script>
</body>
</html>